Toxic or nuclear contamination of soil and its migration within the soil is a continuing threat to the environment, particularly the water supply. The contamination could result from surface spills, pesticide spraying, disintegration of metal containers of discarded chemicals or oils buried in the ground, nuclear accident, and general industrial neglect in disposal of solvents, polychlorinated biphenyls (PCB's) and other commercial waste.
U.S. Pat. No. 4,778,628 (Saha et al.) describes an underground waste barrier structure constructed by forming a recessed area within the earth, lining the recessed area with an outer layer of clay, a middle layer of zeolite and an inner layer of activated carbonaceous material into which the waste, in the form of 55 gallon steel drums containing toxic material can be placed. A ceiling is then formed of the layer materials, and the whole covered with earth. This and other similar methods did not address problems of free, in-situ toxic or nuclear contamination in the soil.
In U.S. Pat. No. 3,559,737 (Ralstin et al.) a method to limit or control leakage or spill conditions in volatile hydrocarbon fuel storage reservoirs in the earth is discussed. Here, localized, in-situ freezing is used to form an impervious cryogenic structure in caprock fractures and frozen barriers in topographical saddle regions. A series of wells are drilled to provide a line of wells through which refrigerant is circulated to freeze the earth and form frozen sections; using techniques developed earlier, for forming frozen containment barriers to enclose storage reservoirs, and for forming frozen foundations within the ground, as taught in U.S. Pat. Nos. 3,183,675 (Schroeder) and 3,267,680 (Schlumberger), respectively. Using similar technologies, U.S. Pat. No. 4,597,444 (Hutchinson) teaches freezing an entire volume of soil about oil recovery shafts drilled into the earth, to prevent collapse of the shaft sides, by using refrigerant injecting shafts at selected distances outside the diameter of the oil recovery shaft.
In U.S. Pat. Nos. 4,860,544; 4,974,425; and 5,050,386 (all Krieg et al.) all the prior techniques were used to provide a method for complete containment of hazardous material in the ground, such as gasoline, oil, PCB's, radioactive isotopes, or a garbage dump, by providing an enclosing, flow-impervious cryogenic barrier. These patents teach an array of boreholes extending downward from spaced apart locations on the periphery of the containment site. A flow of refrigerant medium is established in the barrier boreholds whereby water in the portions of the Earth adjacent to the barrier boreholes freezes to establish ice columns extending radially about the boreholes. The lateral separations of the boreholes and the radii of the ice columns are selected so that adjacent ice columns overlap. The overlapping ice columns collectively establish a closed, flow-impervious barrier about the predetermined volume underlying the containment site. The main embodiment utilizes a subterranean, frozen, Vee shaped trough barrier extending into and through subsurface layers below ground level.
The Vee shaped trough barrier taught by Krieg et al. is enclosed on all sides except the top, to completely underlie, encompass, and trap the hazardous material in a manner to allow no natural flow of water into or out of the contained ground. A gas pressure test is used to confirm that the barrier is complete. In the case where there is a shallow fluid-flow impervious sub-stratum, the cryogenic fluid pipes need not converge but may extend from spaced apart locations on the perimeter of the containment surface to that sub-stratum establishing a completely closed "picket fence" like ring of pipes to fully enclose the hazardous volume. Contaminant tends to collect at the bottom of the encompassing barrier where it may be pumped out if desired.
In another embodiment of the Krieg et al. inventions, a predetermined volume within the containment system is frozen, while other predetermined volumes are kept unfrozen in a manner such that a frozen cell portion can be lifted out of the ground from its in-situ position. This cell can be sprayed with water to establish an ice glaze on all sides, to prevent hazardous material on the cell periphery from becoming windborne, after which the cell can be removed from the site. This process of hazardous material removal would involve moving large volumes of earth.
There are instances where it is advantageous not to completely enclose a mass of hazardous material in the ground, but rather direct flow of the hazardous material with groundwater in a controlled manner to concentrate the hazardous material at a remote location for further treatment, without massive earth removal. It is one of the objects of this invention to provide such a method of controlled flow, concentration and treatment.
Accordingly, the invention broadly resides in a method of treating flowable contaminants beneath a surface region in the ground, characterized by freezing at least one, open subsurface barrier volume of earth in the form of a wall structure near the contaminated region, and positioning the frozen barrier such that the flow of contaminants is directed along a predetermined, subsurface path, causing concentration of contaminants. The barrier can be a single wall of frozen earth, a plurality of walls of frozen earth directing a flow of contaminants between them, a partly surrounding connecting walled structure, or the like. The barrier is frozen by techniques well known in the art, such as using closely spaced concentric pipes to pump or otherwise pass and recycle a refrigerant medium through the ground within the volume of earth where the frozen barrier is desired. The circulating refrigerant removes heat from the ground around the pipes and forms cylinders of frozen ground, which cylinders eventually join without any gaps to form a wall of frozen ground.
The invention additionally resides in a method of treating flowable contaminates beneath a surface region in the ground, characterized by: (A) freezing a subsurface barrier volume of earth having two end portions and a central portion partly surrounding the contaminated region and vertically extending to an impervious strata layer; (B) positioning the frozen barrier to allow a flow of contaminants within the subsurface barrier through an opening between the two end portions of the barrier in a manner to concentrate the contaminants; and (C) concentrating the contaminants. In one embodiment, the flow of contaminants will be from inside the barrier, and through a narrow concentrating opening between the two end portions, to a point outside the barrier. In another embodiment, the flow of contaminants will be toward the central portion of the barrier and between a wide opening between the two end portions to cause concentration near the central portion. A variety of remediation means, such as resin columns, electrokinetic anodes and cathodes, and the like can be disposed within the concentration side and a variety of chemicals can be pumped into, or water pumped out of the contaminated subsurface volume.
This method of this invention has the advantages of both directing and concentrating contaminates while actively and efficiently performing remediation. The contaminants will usually be in the form of hazardous or nuclear contaminants. The ability to direct contaminant migration with frozen barriers provides the means to make feasible large scale remediation which heretofore has been impractical, uncontrollable, and prohibitively expensive. By utilizing this application, the subsurface contamination can be redirected from a sensitive area, such as an aquifer, river, or stream, etc. and then remediated in a capture zone. This is extremely important, for example, in controlling a plume. The redirection of subsurface contaminants is possible because the diffusion rate of hazardous and nuclear contaminants through frozen soil barriers is extremely low. Frozen barriers also provide a flexible, economical, and removable method to effectively direct the path of contaminants in the earth and have the added feature of being self-healing.
Conventional cryogenic approaches to containment are limited in their approach: they either form a "bathtub" around the contaminated area with an array of vertical and slanted freeze pipes, or they form a completely enclosing vertical wall with freeze pipes in a "picket fence" arrangement when there is an impervious soil layer close to the surface around the contaminated area. In either case, the conventional approach is to form a totally closed barrier around the entire periphery of the contaminated site.
There are a significant number of applications where it is unnecessary and/or uneconomical to totally enclose a contaminated volume, as taught in the prior art, to achieve effective containment. For example, topographical and subsurface soil conditions may exist such that there may be a relatively shallow unconfined aquifer spread over a large area that discharges groundwater to a stream. To achieve effective containment, a frozen barrier along the stream would be sufficient to control contaminant migration as long as the upstream groundwater was eliminated with wells or some type of drainage system. Simultaneously, remediation technologies could be directed at the containment area, such as electrokinetics with a recycle system to inject lixiviants or a pump and treat system. Similarly, many of the government burial grounds contain unlined trenches. In order to excavate these trenches (where there may or may not be contained water) which usually parallel one another, a frozen barrier that acts as a retaining wall, prevents hazardous/nuclear cross contamination, minimizes excavation, and is an ideal short term application for an open frozen barrier.
Other examples of applications of this invention include remediation efforts entailing excavation of the contents of a particular trench in a group of trenches where there is very shallow impermeable bedrock that is, for instance, located downgrade along a slope. It would only be necessary to construct a frozen barrier around three sides of the trench to the impermeable layer, since the contents would be recovered from the fourth, and uphill, side. This invention can be utilized in numerous other applications to contain contaminant plumes in sites not currently addressed by existing frozen, "closed barrier" barrier technology, such as long sloping sites where groundwater must be controlled or large areas where perhaps there are burial trenches in sloped sites containing groundwater.